Answer
$7.9 \ units$
Work Step by Step
Let $C$ be the circumference of a circle:
$C=2 \pi r$ or, $C=\pi d~~~~(1)$
Here, $r$ represents radius and $d$ is diameter.
The endpoints of the semi-circle correspond to the endpoints of its diameter. We use the distance formula to find:
$d=\sqrt{(4-1)^2+(7-3)^2}=\sqrt{25}=5$
Thus, equation (1) becomes:
$L=\dfrac{1}{2} \pi d=\dfrac{1}{2} \times \pi (5) \approx 7.9 \ units$
